1. Field of the Invention
This invention pertains to total artificial hearts (TAH) which are self contained for permanent emplacement within a living being for total replacement of a natural heart. More particularly, this invention relates to a TAH which utilizes a hydraulic pumping system with a small, reversible pump to actuate pumping action.
2. Prior Art
Despite the climbing mortality rate of persons who die annually because of the limited supply of heart transplants, there are currently no commercial artificial hearts available for patient use. It is estimated that 35,000 people per year in the United States alone need a replacement for the irreparable natural heart. Because only 2,000 donor hearts are available on the average, the vast majority (33,000) of these heart patients will die. By 1995, it is estimated that the target population for circulation support devices will be 60,300. Artificial Heart, Institute of Medicine, Jul. 23, 1991 (advance copy).
Numerous patents have been issued which offer various technologies intended to address this critical need. These technologies extend from ventricle assist devices to the fully self contained TAH. U.S. Pat. No. 4,173,796 issued to the present applicant in 1979 introduced the concept of a permanent electrohydraulic heart capable of being implanted within a living body with pumping support being provided by a reversible, electric impeller motor. The theory of operation for this device was to place the motor at the base of the TAH and reversibly transfer hydraulic pumping fluid between respective pumping chambers of the left and right ventricles. The motor was situated below the blood chambers and was separated by means of channels.
Current commercial artificial heart devices are limited to air driven hearts whose cost approaches $100,000 per TAH and supporting air drive system, not counting associated medical fees for emplacement. Because of this high cost, each consideration of use of a TAH remains primarily a financial issue. Until such costs can be contained, it is apparent that artificial hearts will remain a theoretical solution to a real life problem.
Numerous problems encumbered the intended success of the electrohydraulic heart invented by Jarvik and issued to the present applicant in the patent referenced above. The capacity of the motor was limited by its conventional design. Such design features included the concept of a central rotating shaft with attached impeller blades radiating outward from this shaft. The shaft was rotated by a brushless DC motor in a conventional manner. Fluid flow was advanced through an annular channel in a reversible manner, based on the direction of shaft rotation.
Response time of the hydraulic fluid was not optimal. This was perhaps a byproduct of several design features. For example, the annular channel configuration imposed a substantial amount of surface area and frictional drag on the transferring fluid between pumping chambers of the respective ventricles. In addition to the drag induced by the large surface area of the flow channel within the motor, there was further drag developed by the substantial distance of travel required of the fluid as it moved from one pumping chamber to the other. This was a byproduct of the placement of the pump below the pumping chambers providing a flow channel to direct the flow between the left and right ventricles. Separate wall structure was provided below the septum to isolate the pump motor and define the described flow channel. Such a tortuous path for fluid movement which is being directionally reversed as much as 120 times per minute posed a major obstacle to confidence in the long term survivability of the TAH.
What is needed is a greatly simplified hydraulic fluid transfer system which optimizes rates of fluid transfer and minimizes drag, without perpetuating the former high cost of production and emplacement.